1. Field of the Invention
The invention relates to an organic electroluminescence device which obtains high-luminance and reduced unevenness in luminance, and in particular, to an organic electroluminescence device of multi-photon emission mode, showing little unevenness in luminance even in a large-area format electroluminescence device.
2. Description of the Related Art
Organic electroluminescence devices (which are referred to hereinafter as an “organic EL device” in some cases) containing a thin film material that emits light by excitation due to application of electric current are known. The organic electroluminescence devices, which obtain high-luminance light emission at low voltage, have broad potential applications in fields such as cellular phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination, and also have advantages of reducing the thickness, weight, size, and power consumption of the devices in the respective fields. Accordingly, such devices are greatly expected to become the leading devices in the future electronic display market. However, there are still many technical problems to overcome, such as with respect to luminance and color tone, durability under various ambient operating conditions, and mass productivity at low cost, in order for these devices to be practically used in these fields in place of conventional display devices.
Even higher luminance is needed, depending on the application. Various studies aimed at improvement in the quantum efficiency of luminescence devices and development of a method of extracting emitted light outward at high efficiency are now in progress for improvement in luminance.
Japanese Patent Application Laid-Open (JP-A) Nos. 2003-272860 and 2004-39617 disclose, as a means for increasing luminance, a device of multi-photon emission mode which includes plural laminated light emission layers (each light emission layer is referred to hereinafter as a “unit light emission layer” in some cases), wherein the unit light emission layers each independently include a light emission layer and an auxiliary layer, and are connected to each other by a charge generation layer. The charge generation layer includes an electric insulating layer formed by an n-doped layer and a p-doped layer. However, in a case where an electric insulating layer is provided as the charge generation layer, a loss of electroluminescent quantum yield with respect to current is unavoidable, and thus results in a disadvantage of lowering emission luminance. Also, an unevenness in the luminance induced by a voltage drop increases and presents a major problem particularly in the case of preparing a large-area format electroluminescence device.
Also, JP-A Nos. 11-329748 and 2003-45676 disclose a structure including an electrically conductive layer as the charge generation layer. However, it is difficult, in such a structure, to exhibit a sufficient multi-photon emission effect.